Traces of olefinic impurities are often found in carboxylic acids or their anhydrides. These olefinic impurities are either introduced as a result of their application and recycle or as a consequence of their method of manufacture.
As a point of illustration, long chain saturated carboxylic acids generally contain small quantities of the olefin from which they were generated. For example, nanoic (C9) acid generated by hydroformylation and oxidation might contain 1-octene. These olefins are difficult to separate and, generally speaking, the low levels (ca.&lt;100 ppm) of these olefinic impurities are tolerated commercially in the longer chain carboxylic acids. However, when subsequently used to generate carboxylic anhydrides by the exchange of acetic anhydride, the acetic acid by-product is also contaminated with the olefin. This problem is compounded by the fact that these olefins commonly distill as azeotropes (i.e., as azeotropic pairs) with the acetic acid.
As a further example, during the production or coproduction of acetic acid and acetic anhydride via the carbonylation of methyl acetate, acetone is generated as a by-product. This acetone by-product leads to the generation of mesityl oxide and 3-methyl butenoate derivatives by a condensation reaction.
These by-products may be difficult to remove. Generally, these small quantities of olefin might be regarded as unimportant. However, a critical parameter in achieving commercial grade carboxylic acids, particularly for C2 through C4 carboxylic acids, is the so-called "substances reducing permanganate" test. This test is adversely affected by the presence of these olefinic contaminants and reductions in the olefin level are often required when highly pure, or at least olefin-free, carboxylic acids and anhydrides are desired.